Purifying electrolytes.



ST TES PATENT GFFICE.

FRANZ vON xficnncnn AND FRITZ voN BIDDER, or HOLCOMBS nocx, VIRGINIA,AND GEORGE O. SEWARD," or E ST-ORANGE, NEW JERSEY, ASSIGNORS TO VIRGINIALABORATORY comrAN r, or new YORK, N. Y., A CORPORATION or NEW YORK.

PU'BIFYING ELEGTBOBYTES.

Specification of Letters Patent.

Patented May 5, 1914.

No Drawing. Application filed September 16, 1908, Serial No. 458,259.Renewed November 21, 1918.

' Serial No. 802,326.

specification.

In the electrolytic production of light metals from their fusedsalts,the presenceof even small quantities of impurities in theelectrolyte leads to trouble in a continuous run if such impurities aresalts of less electro-positive metals than the metal sought and which donot allo with the desired metal. Such metals fal to the bottom of theelectrolytic vessel after being decomposed and form there a conductivemass. Besides such decomposable metallic impurities, there occur in thecommercial salt ofv the metal sought also small quantities of compoundswhich do not dissolve in the electrolyte, but settle down to the bottom.

These are generally pyroconductive mate-- rials, so that the result in along run is the formation of a partly metallic conductive layer aroundthe upwardly projectin submerged cathode (which we preferab y-use insuch work), which has a decidedly injurious effect on the electrolysis.This is particularly noted in the production of alkaliearth metals,which demand in their production a high current density at the cathode.Such d conductive layer around the cathode will reduce the currentdensity more and more as it accumulates. When such an accumulation hasgone to a certain extent, the earth alkali metal sought commences toseparate in a spongy state, instead of molten, and finally theseparation of metal at the cathode ceases entirely. The same phenomenonoccurs in the electrolysis of alkali chlorlds, though the effect is notso pronounced nor so quickly fatal.

The present invention provides a method of removing such objectionableimpurities from the electrolyte prior to electrolysis and consists in apartly mechanical, partly electrical, purification of the salt of themetal sought before introduction into the electrolytic vessel.

As an illustration of our process, we will describe the purification ofsodium chlorid prior to electrolysis,

The commercial salt is melted in an elec-,

tric furnace with carbon electrodes in a crucible composed of thechilled salt itself, the current (preferably alternating current) beingbrought in through. one electrode and out throughthe other so that thesalt is maintained molten for only a controllable distance around theelectrodes, the material of the shell or case of the melting cruciblenot coming at any time in contact with the molten salt. By maintaining aconvenient port-ion of the salt molten for a certain time, themechanical impurities in the salt settle out therefrom, and clear andalmost entirely purified salt could be ladled out from the furnace, or,if the melting furnace was arranged to tilt, the salt could be poured inpurified form therefrom.

In the purification of sodium chlorid, such a mechanical purification isnearly ideal, but we find that there still remain certain impurities insolution which cannot settle out .mechanically while maintaining thechlorid molten by alternating current. Such impurities are then removedby replacing the alternating current by direct current for a shortwhile. Being lesselectrO-positive. they are decomposed beforethesettling the mechanical impurities in a melting furnace operated byalternating current, and then removing the partly or odes so that thevoltage is high and of lowintensity, the less electropositive'impurities are-decomposed, while the metal sought is separated only to alimited extent.

A convenient way of employing the direct current alone would be to useas a cathode in the melting furnace a heavy metal which would bemaintained molten and which WOuldtbSO-Ib the impurities and the smallamount of sodium produced after the impurities are separated out. Fdrexample, lead could be used as the molten cathode in purifying sodiumchlorid. Another variation of our process would be to maintain the saltto be purified molten with alternating current and keep in contact withitsome of the more electro-positive metal sought, which would decomposethe less electropositive impurities, the metals from which would thensettle to the bottom. In the purification'of sodium chlorid, forexample, we would melt the salt in a vessel with the alternating currentand keep metallic sodium in contact with the molten salt, protecting thesodium from the air by a suitable cover, which being remote from thesource of heat and hence much cooler than the chlorid; prevents loss ofthe sodium by evaporation. Whichever variation of our process is used,the result is the same-we obtain from a more or less impure commercialsalt a perfectly. pure salt freed from its mechanical and dissolvedimpurities and suitable for feeding into the electrolytic vessel-;1nwhich the metal of that salt is separated by the direct current.

In many cases the mere remelting and mechanical settling out of theimpurities will be suficient; in other cases both the mechanical andelectrolytic purification will be necessary. It depends on the nature ofthe commercial salt used.

While we have specifically mentioned the chlorids as being susceptibleto purification by our process, we do not limit ourselves to chlorids.Other halogen compounds may be treated in the same manner.

We consider our process of particular ad- Van'tagein themanufacture ofalkali and alkali-earth metals, as we have found that a continuouselectrolysis can not be carried on unless such preliminary purificationis adopted. An incidental advantage of our process is that it dehydratesthe material at p the same time, so obviating the necessityofdecomposing the moisture present in the salt in the electrolytic vessel.

What we claim is:-

1. In the electrolysis of molten salts, fusing the salt and treating itwith a suitable current until the heavier impurities precipitate, thentreating the molten salt with direct current to decompose the lesselectropositive impurities, until the foreign metals thereby separatedprecipitate, and thereafter decomposing the purified salt byelectrolysis.

2. In the electrolysis of molten sodium chlorid, fusing the salt andtreating it with a suitable current while maintaining over it a poo-l ofsodium, until the heavier impurities precipitate, then treating themolten salt with direct current to decomposethe less electro-positiveimpurities, until the foreign metals thereby separated precipitate, andthereafter decomposing the purified salt by electrolysis.

3. In the electrolysis of molten salts, fusing the salt by a currentbetween electrodes while the fused salt is held in a crucible of thesolid salt, maintaining a suitable current until the heavier impuritiesprecipitate, then treating the molten salt with direct current todecompose the less electro-positive impurities, until the foreign metalsthereby separated precipitate, and thereafter decomposing the purifiedsalt by electrolysis.

4. In the electrolysis of molten salts, purifying the salt by fusing itby a current be tween electrodes while the fused salt is held in acrucible of the solid salt, maintaining a suitable current until theheavier impurities are precipitated, and thereafter decompose ing thepurified salt by electrolysis.

5. In the electrolysis of molten salts, fusing the salt and treating itwith a suitable current between an anode and a molten cathode of heavymetal until the heavier impurities are precipitated and absorbed by suchmolten cathode, and thereafter decomposing the purified salt byelectrolysis.

6. In, the electrolysis of sodium chlorid, treating the molten salt witha suitable current while maintaining over it a pool of sodium, until theheavier impurities precipitate, and thereafter decomposing the purifiedsalt by electrolysis.

7. In the electrolysis of-sodium chlorid, treating the, molten salt witha suitable a1 ternatin current until the heavier impurities precipitate,and thereafter decomposing the purified salt by electrolysis.

8. In the electrolysis of molten salts, purifying the salt by treatingit first with an alternating current and afterward with a directcurrent, to precipitate the heavier impurities, and thereafterdecomposing the purified salt by electrolysis. 1

9. In the electrolysis of sodium chlorid, treating the molten salt firstwith an alternating current and afterward with a direct current, toprecipitate the heavier impurities, and thereafter decomposing thepurified salt by electrolysis.

In witness whereof, we

have hereunto signed our names in the presence of two sub-- scribingwitnesses.

Witnesses as to Franz Von Kiigelgen and Fritz Von Bidderi HARRY R. LEE,J. H. WEBB. Witnesses as to George 0. Seward:

FRED WHITE, THEODORE T. SNELL.

